The physiological processes intervening between the absorption of light by a photopigment and the generation of an electrical signal by the primary photoreceptor cell are not yet known. We propose to study three parts of this problem: the biophysical and ionic basis of the coupling between photopigment and the generation of the receptor potential; the ionic bases of the generation of the receptor potential itself; and the propagation of signals from photoreceptor cells to higher-order neurons. Specifically, we propose to study the ion flux changes which accompany photoreception in the Limulus ventral photoreceptor cells. We propose to study the ion fluxes of the photoreceptors as functions of both light and membrane potential. To do this we will employ both radioisotope tracers and microelectrode electrophysiological techniques. This should allow more extensive interpretation of the mechanisms whereby these cells transduce light into voltage changes, than has been afforded by electrical measurements alone. We also propose to continue to study the ionic bases of the generation of receptor potentials and the adaptation processes in vertebrate photoreceptors by intracellular microelectrode techniques. Some of our evidence supports the hypothesis that intracellular calcium ions mediate the light-induced reduction of sodium conductance of the rod cell membrane.